Permanent implantation of either iodine-125 seeds or palladium-103 radioactive seeds in the prostate has become a popular form of radiation therapy for carefully selected prostate cancer patients. The procedure is minimally invasive and is usually performed in a one-day surgery unit on an outpatient basis. The treatment related morbidity such as urinary incontinence and sexual impotence is fairly low for seed implantation, and control of the cancer in selected patients is as good as that with other treatment modalities such as surgery. For these reasons the public interest in seed implantation is growing rapidly. The clinical success of radiation therapy has been highly dependent on the capability of delivering a desired prescription dose. For prostate seed implant, such a capability is impinged on both the accuracy of placing the radioactive seeds in a predesigned spatial pattern for adequate spatial dose coverage and on the accuracy of the fundamental dosimetry properties of each individual seed. The primary objective of this project is to develop a new method for determination of dose rate constant, the only absolute quantity in the AAPM TG-43 formalism, for interstitial brachytherapy seeds that are used for prostate seed implants. The new method utilizes high-resolution gamma ray spectrometry and would be capable of determining the dose rate constant without the need of knowing the air kerma strength. Our hypothesis is that the proposed new method based on gamma ray spectrometry is superior to currently accepted standard experimental method using LiF TLDs. It has the potential of improving the accuracy and consistency of the dosimetry of prostate seed implants. This improvement in absolute dosimetry will impact the clinical practice of all brachytherapy patients because unlike random errors, errors in absolute dosimetry parameters for different models of brachytherapy sources affect all patients in a systematic fashion and in the same direction. Since prostate brachytherapy has now become a treatment method of choice for selected patients and this popularity has led to the introduction of many new designs of seeds for clinical implementation, we hypothesize that gamma ray spectroscopy can be used to reduce the uncertainty in dose rate constants and improve the dosimetry in a large number of prostate cancer patients.